1. Field of the Invention
The present invention relates to a gas feed system for a diesel engine, and more particularly to a gas feed system for feeding gas to a high-pressure gas injection-diffusion combustion type diesel engine.
2. Description of the Prior Art
With regard to diesel engines making use of gas as a fuel, a low-pressure gas premixing combustion type and a high-pressure gas injection-diffusion combustion type are known, and in the prior art the low-pressure gas premixing combustion type is employed in many cases. This type of diesel engine makes use of a method in which a premixed fuel-air mixture is sucked in during an intake stroke or gas is injected into a cylinder from the initial period to the middle period of a compression stroke. With such a method abnormal combustion is liable to be generated due to a temperature rise caused by adiabatic compression during the compression stroke. Hence, to employ such a method the compression ratio must be made lower than in a diesel engine operated by oil fuel, and accordingly, thermal efficiency and output power are lowered.
Therefore, the developments of high-pressure gas injection-diffusion combustion type diesel engines is being carried forward by various companies at present. FIG. 4 shows one example of this high-pressure gas injection-diffusion combustion type of diesel engine in the prior art. With reference to this figure, gas boiled off from a liquefied natural gas tank 11 is pressurized by a high-pressure compressing apparatus 30, it is cooled down to an ordinary temperature by sea water in a heat-exchanger 40, and then it is fed to a diesel engine 70 through a high-pressure gas feed pipe 41. The high-pressure compressing apparatus 30 consists of a 4-stage reciprocating-type compressing mechanisms 31, 32, 33 and 34, which are driven by an electric motor 35. Between the third stage compressing mechanism 33 and the fourth stage compressing mechanism 34 is provided an intercooler 36 in which pressurized gas is cooled by sea water. It is to be noted that reference numeral 10 designates a liquefied gas tank installation.
However, in the high-pressure gas injection-diffusion combustion type diesel engine, a lot of power is necessary for pressurzing gas to a high pressure. In one trial example in which a liquefied natural gas tank having a capacity of 125,000 m.sup.3 and, 0.1% of the total amount of liquefied gas therein boils off per one day the calculated, compressor power necessary for pressurizing the total amount of liquefied gas to 250 bars (atms.) is about 700 kW.